Typical protocols call for inspection of welds within a nuclear reactor during scheduled outages. Inspection devices are used to scan the welds to detect intergranular stress corrosion cracking (IGSCC), which has been known to occur for example, in connection with primary system piping dissimilar metal (DM) welds.
Circumferential welds extend around the periphery of cylindrical conduits, such as the pipes and tubes commonly found in a reactor system. These welds may be inspected by positioning a scanning apparatus proximate to an outer surface of a pipe adjacent to the location of a weld, and rotating the scanning apparatus about the circumference of the pipe to inspect the entire extent of the weld. The inspection cannot be thorough and precise if the apparatus drifts axially so as to uncontrollably spiral or shift as it rotates around the pipe. In fact, even if the scanning apparatus is designed to follow a helical path, axial drift significantly increases the risk of missing defects in regions that escape inspection.
Previous attempts to address the problem include systems that detect drift and correcting axial alignment reactively by steering the instrument to compensate for detected drift. However, detection of drift necessarily means that the drift has occurred, so the resulting inaccuracy has already impacted the precision of the inspection. Other attempts to prevent drift involve driving the scanning apparatus along an external track that is substantially fixedly mounted around the circumference of the conduit. Wheels and gears engage the external track such that the scanner rides on the external track rather than on the surface of the conduit. However, riding above the surface of the conduit on the external track elevates the profile of the scanning apparatus, which limits its ability to operate in areas with limited clearance. To achieve axial translation, the transducer element of the scanning apparatus typically articulates axially along an extension arm that extends away from the external track. An extended arm often fails to maintain stable and consistent contact or proximity between the scanning apparatus and the conduit, particularly when scanning the underside of a conduit, which results in hysteresis.
Achieving the desired level of precision is especially difficult when inspecting a tapered pipe that has diameters that vary along its length, or when using the same apparatus to inspect different various diameter pipes. There is a need, therefore, for systems and methods that prevent axial drift, and that are usable with various sized conduit and as well as with tapered conduit.